Survival rates in patients with acute myeloid leukemia (AML) are low, especially in the elderly (median survival &lt; 1 year) in whom the disease is most common. Moreover, treatments are toxic, confining patients to hospitals and requiring frequent clinic visits. Thus, new approaches to treatment are needed to improve both outcomes and quality of life. With these issues in mind, an international team of investigators led by Arthur Zelent from the Institute of Cancer Research, UK, has tackled the question of how to induce leukemia cells, which are arrested at an immature stage, to differentiate. The inspiration for these studies came from the success of all-trans-retinoic-acid (ATRA) in the treatment of acute promyelocytic leukemia (APL). Outcomes using ATRA for APL elegantly demonstrated that differentiation-based therapy can have a high “therapeutic index,” meaning the therapeutic agent produces significant benefit with little toxicity. ATRA has been tried unsuccessfully in the treatment of other subtypes of AML. Therefore, Zelent’s group set out to characterize the mechanism by which AML cells resist ATRA-induced differentiation and in the process showed that the resistance to differentiation can be reversed by using a commercially available antidepressant drug.

Leukemic blasts are characterized by increased proliferation and lack of differentiation. These qualities are acquired through both genetic and epigenetic mechanisms. Epigenetic changes include modifications of DNA and histones (complexes of proteins that comprise the nucleosomes of chromatin that packages DNA). Different histone modifications support either gene expression or repression. For example, dimethylation of histone H3 at lysine 4 (H3K4me2) is observed in the promoter region of actively expressed genes, and expression of these genes is turned off (repressed) by lysine-specific demethylase 1 (LSD1) that converts dimethylated H3K4 (H3K4me2) to mono- (H3K4me) or unmethylated H3K4. Of relevance to the current study, tranylcypromine (TCP), an LSD1 histone demethylase inhibitor, is marketed as an antidepressant and is available commercially.

For ATRA to be effective it must bind to its nuclear receptor called retinoic acid receptor (RAR), and the ATRA/RAR target genes must be in the “on” position so that expression can be induced by the ATRA/RAR transcription factor complex. Prior studies have shown that some ATRA-responsive genes are inactive in leukemia as a consequence of histone modification. In the current study, Schenk and colleagues used ATRA in combination with LSD1 inhibitors to induce differentiation by derepressing epigenetically silenced genes. Synergy between ATRA and LSD1 inhibitors (including TCP) to induce myeloid differentiation of several leukemia cell lines was observed in vitro. To determine whether the combination might be therapeutic in “real-life” AML, Schenk et al. assessed the effect of ATRA/TCP in primary AML cells from three patients using a murine xenotransplant model. They reported that pretreatment of AML samples with ATRA/TCP followed by in vivo administration of the drugs after leukemic cell injection prevented AML expansion. Importantly, there was no effect of ATRA/TCP on engraftment of normal cord blood, suggesting that this combination therapy would likely have a favorable therapeutic index. To test the capacity of ATRA/TCP to treat leukemia once it has developed (mimicking the clinic situation), primary AML samples from the three patients were injected into mice and allowed to expand for 15 days. Administration of ATRA/TCP significantly reduced the tumor burden in each of the three leukemic mouse models. Interestingly, in two of these AML models, ATRA alone had an effect similar to the ATRA/TCP combination.

To investigate the hypothesis that TCP paves the way for ATRA to activate gene expression by histone modification, the investigators assessed how the combination of ATRA/TCP affects gene expression and H3K4 methylation. A strong correlation was observed between gene expression and H3K4me2 in promoter regions of specific genes, some of which (e.g., CD11b, CD11c, and CD85) are associated with myeloid differentiation.

This exciting preclinical study suggests that epigenetic modifiers in combination with differentiation therapy may be an effective approach to treating AML. We anticipate that additional preclinical studies will further characterize the basis of the resistance to ATRA-induced differentiation and thereby provide the underpinning for the optimal use of this low-toxicity approach to the treatment of AML. In this way, the depression surrounding AML treatment may be mitigated by derepression.